1. A partial mechanistic understanding of the North
American monsoon
Ehsan Erfani1,2
and David Mitchell1
1
Desert Research Institute, Reno, Nevada, USA, 2
Graduate Program in Atmospheric Sciences, University of Nevada, Reno,
Nevada, USA
Abstract An understanding of the major governing processes of North American monsoon (NAM) is
necessary to guide improvement in global and regional climate modeling of the NAM, as well as NAM’s
impacts on the summer circulation, precipitation, and drought over North America. A mechanistic understanding
of the NAM is suggested by incorporating local- and synoptic-scale processes. The local-scale mechanism
describes the effect of the temperature inversion over the Gulf of California (GC) on controlling low-level
moisture during the 2004 NAM. The strong low-level inversion inhibits the exchange between the moist air in the
marine boundary layer (MBL) and the overlying dry air. This inversion weakens with increasing sea surface
temperatures (SSTs) in GC and generally disappears once SSTs exceed 29.5°C, allowing the moist air, trapped in
the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported by across-gulf
(along-gulf) flow toward the NAM core region (southwestern U.S.) to form thunderstorms. On the synoptic
scale, climatologies from 1983 to 2010 exhibit a temporal correspondence between coastal warm tropical surface
water, NAM deep convection, NAM anticyclone center, and NAM-induced strong descent. A hypothesis is
proposed to explain this correspondence, based on limited soundings at the GC entrance (suggesting this local
mechanism may also be active in that region), the climatologies, and the relevant literature. The warmest SSTs
moving up the coast may initiate NAM convection and atmospheric heating, advancing the position of the
anticyclone and the region of descent northward.
1. Introduction
1.1. Characteristics of the North American Monsoon
The phenomenon referred to as the Mexican monsoon, Arizona monsoon, or North American monsoon
(NAM) is a weather pattern responsible for significant rainfall during the summer (July–September) over
northwestern Mexico and the southwestern United States. It supplies about 60%–80%, 45% and 35%
of the annual precipitation for northwestern Mexico, New Mexico (NM), and Arizona (AZ), respectively
[Douglas et al., 1993; Higgins et al., 1999; Mitchell et al., 2002; henceforth M2002]. The NAM is initiated by
deep convection and heavy precipitation over the western coast and the Sierra Madre Occidental (SMO)
mountains of central Mexico typically around mid-June (just north of Puerto Vallarta-Guadalajara region), and
then propagates northwestward along the western slopes of the SMO, finally reaching its northern terminus
in the United States around mid-July, generally in the Great Basin and the Arizona-New Mexico (AZNM)
region [Douglas et al., 1993; Higgins et al., 1999].
NAM rainfall is associated with the amplification and northward shift of the upper level anticyclone over
the southwestern U.S., called the monsoon anticyclone or monsoon high [Carleton et al., 1990; Higgins et al.,
1998, 1999]. The monsoon anticyclone forms in mid-May over the southern Mexico and strengthens and
migrates northward during the summer [Higgins et al., 1999]. The NAM anticyclone is linked to a low-level
thermal low that forms over the desert southwest and is dominant from mid-June to mid-September and
extends vertically up to 700 hPa over AZ in July through early August [Rowson and Colucci, 1992]. Enhanced
NAM rainfall is associated with rising motion south of the NAM anticyclone, and reduced rainfall coincides
with subsidence north and east of the NAM anticyclone [Higgins et al., 1997].
Much of the NAM literature concludes that the eastern tropical Pacific and/or Gulf of California (GC) are
the major sources of moisture for the NAM [Hales, 1974; Douglas, 1995; Stensrud et al., 1995; Anderson and
Roads, 2001; Berbery, 2001; Bordoni and Stevens, 2006; Barron et al., 2012]. Reitan [1957] showed that the
greatest amount of precipitable water during the monsoon is found between the surface and 800 hPa and
that the Gulf of Mexico (GM) is unlikely to be the source of this low-level moisture, considering the
ERFANI AND MITCHELL ©2014. American Geophysical Union. All Rights Reserved. 13,096
PUBLICATIONS
Journal of Geophysical Research: Atmospheres
RESEARCH ARTICLE
10.1002/2014JD022038
Key Points:
• Inversion over Gulf of California
weakens with increased SST and
allows vertical mixing
• Association between tropical surface
water, monsoon convection,
and anticyclone
Correspondence to:
E. Erfani,
Ehsan.Erfani@dri.edu
Citation:
Erfani, E., and D. Mitchell (2014), A partial
mechanistic understanding of the North
American monsoon, J. Geophys. Res.
Atmos., 119, 13,096–13,115, doi:10.1002/
2014JD022038.
Received 14 MAY 2014
Accepted 6 NOV 2014
Accepted article online 11 NOV 2014
Published online 4 DEC 2014

2. topography between AZ and the GM exceeds 800 hPa. Due to less moisture content over the GM compared
to western Mexico at lower to midlevels, Douglas et al. [1993] concluded that easterly flow and horizontal
transport of GM moisture does not explain the high moisture over the SMO at these levels. Hales [1972]
relates the importance of the GC to its geographical features as a natural channel surrounded by the SMO in
the east and the mountains of Baja California Peninsula in the west. That is, the GC is a channel with a width
of ~320 km and a length of ~1100 km, which facilitates the low-level transport of moisture from the
tropical eastern Pacific to the southwestern U.S. Similarly, it may also be viewed as a channel for surface
ocean heat transport from the tropics to midlatitudes [Castro et al., 1994; Ripa, 1997; Beier, 1997], with
subsequent latent heat absorption by the surface air during the NAM, as suggested by M2002.
The surface winds over the GC show the seasonal reversal from northerly to southerly at the beginning
of summer [Badan-Dangon et al., 1991]. Douglas [1995] utilized field data from the Southwest Area Monsoon
Project conducted during the summer of 1990 (SWAMP-90) and documented a persistent southeasterly
low-level jet (LLJ) directed along the GC axis. Its maximum value was 15 m sÀ1
and was located approximately
300 m above the surface. The observational study of Douglas et al. [1998] based on SWAMP-90 indicated
a nocturnal southerly LLJ extending from the northern GC to the southwestern AZ desert, having maximum
winds in the early morning. Such LLJs can play an important role in the NAM by directing moisture from
the tropical Pacific and/or GC northwestward and then northeastward into the southwestern U.S. Using
the Regional Spectral Model (RSM) from the National Center for Environmental Prediction (NCEP), Anderson
et al. [2002] reproduced these summertime LLJs.
One important feature in the NAM variability is the moisture surge or gulf surge, a coastally trapped wave
that is initiated by a tropical easterly wave (TEW) that crosses near the GC entrance and is then propagated
northwestward along the GC axis [Stensrud et al., 1997; Adams and Stensrud, 2007]. Gulf surges transport
low-level moisture from the tropical eastern Pacific to the southwestern U.S. within 2–3 days during the
summertime and are associated with strong low-level southeasterly winds, low temperatures, rising sea level
pressure, and rising humidity [Brenner, 1974; Stensrud et al., 1997; Douglas and Leal, 2003; Mejia et al., 2010].
A recent paleoclimate study by Barron et al. [2012], using proxy sea surface temperature (SST) records from
ocean sediments, provided evidence for two modes of NAM expression during the Holocene era: (1) less
rainfall over a broader geographical region during the early Holocene (>8000 calendar years before present),
deriving moisture from the Pacific Ocean and GM, and (2) greater rainfall over a more focused region (AZ, NM,
and the Great Basin, similar to present day) during the middle Holocene (~6000 calendar years before
present), deriving moisture from the GC and GM. In (2), GC SSTs are higher and similar to present-day GC SSTs.
This interpretation, which is supported by the results of M2002, may provide clues for the future expression of
the NAM in a warming climate.
Several studies investigated the importance of GC SSTs on NAM rainfall. Using the fourth-generation
Pennsylvania State University-National Center for Atmospheric Research (Penn State-NCAR) Mesoscale
Model (MM4), Stensrud et al. [1995] found that the GC SSTs are critical for realistic NAM simulations. Since the
gridded SSTs from Reynolds and Smith [1994] underestimate GC SSTs, they prescribed a mean July SST value
of 29.5°C throughout the GC to correctly reproduce the NAM. An empirical study of six NAM seasons by
M2002 indicated that no precipitation is observed in the NAM region when GC SSTs do not exceed 26°C. They
also showed that 75% of June–August precipitation in AZNM region occurs 0–7 days after northern GC SSTs
exceed 29°C. The regional modeling experiment by Mo and Juang [2003] used SSTs from Reynolds and
Smith [1994] in the GC and found that GC SSTs had only a small impact on rainfall in the AZNM region.
This appears to be the result of the mentioned underestimation of GC SSTs in the Reynolds and Smith [1994]
SST data. The regional modeling study by Kim et al. [2005] indicated that higher SSTs in the northern GC
increase the rainfall in northwestern Mexico and the AZNM region. However, their simulations show no
dependence of the NAM onset on GC SSTs, which may possibly be due to inadequate treatment of boundary
layer processes in the GC.
The NAM affects not only the southwestern U.S. but also the contiguous U.S. Anomalously wet NAMs in AZ
are strongly correlated with anomalously dry summers in the midwest and somewhat correlated with
relatively wet summers in the southeastern United States [Higgins et al., 1998; M2002]. These correlations are
not well predicted in regional climate models. In a model evaluation study, six regional climate models were
used to analyze regional-scale uncertainties in the context of climate change [Mearns et al., 2012]. The
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3. simulations used boundary conditions from NCEP-Department of Energy (DOE) reanalysis data for a 25 year
period from 1980 to 2004. It was shown that five of these six models (including the WRF model) exhibited
the greatest summer precipitation bias (underpredicted rainfall) in the NAM region. These results suggest
a necessity for bias correction in the NAM region for simulations of future climate. In a NAM model
intercomparison study, Gutzler et al. [2005] found that the Global Climate Models (GCMs) considerably
overestimated precipitation in the core NAM region with a delayed NAM onset relative to observations,
with no significant rainfall west of 112°W (e.g., dry in western half of AZ). Moreover, the simulated peak rainfall
in the NAM core region was in August rather than the observed peak in July. These and other limitations
found in GCM simulations may be due to a failure to resolve the GC and/or GC SSTs and/or an inadequate
treatment of the marine boundary layer (MBL) over the GC.
1.2. GC SSTs and Ocean Currents
Since observational, modeling and paleoclimate studies have implicated GC SSTs as a critical factor in the
development of the NAM, this subsection deals with ocean processes that affect GC SSTs. Moreover, based
on findings in this and other studies, an understanding of how ocean currents affect the GC SSTs may
ultimately prove to be critical in predicting the timing, strength, and regional extent of the NAM. The
following discussion summarizes some oceanography research relevant to GC SSTs and the NAM. As far as we
know, this knowledge has not previously been assembled in a manner that elucidates large-scale ocean
processes that may affect the evolution of the NAM.
The entrance of the GC is a region where two surface currents meet: the southward cool waters of the
California Current (CC) and the northward warm waters of the Mexican Coastal Current (MCC) [Lavín et al.,
2009]. The interaction of these two currents controls the seasonal variation of surface circulation in this
region [Godinez et al., 2010]. The poleward branch of the MCC was considered to be a northward extension of
the Costa Rica Coastal Current (CRCC) during the summer [Wyrtki, 1967]; however, by using historical
data, Kessler [2006] observed no connection between the CRCC and the MCC. Numerical simulations
[Zamudio et al., 2007] and modern observations [Godinez et al., 2010] found that the northward MCC can be
formed locally by the wind stress curl.
Geostrophic currents follow the lines of constant SSH (sea surface height) with stronger currents associated with
steeper SSH gradients. Also, SSH maxima (minima) are associated with anticyclonic (cyclonic) geostrophic
currents [Steward, 2008]. Understanding the seasonal variability of ocean gyres and SSH in the eastern Pacific
Ocean can be useful for shedding light on ocean currents affecting the heat budget of the GC. The CC along
the West Coast of North America is associated with a steep SSH gradient with low SSH along the coast,
bringing Arctic water southward. This SSH gradient is strongest during March and April [Strub and James, 2002];
the lower the coastal SSH is here, the stronger the SSH gradient and CC are.
Figure 1, contributed by Professor Ted Strub from Oregon State University, shows the climatological
evolution of SSH relative to Earth’s geoid (the mean ocean surface if the ocean is at rest) from March through
June, together with the direction of geostrophic currents, for the period 1986 to 1997. During March–April, an
anticyclonic gyre of high SSH is centered in the tropical eastern Pacific at ~12°N latitude southwest of
Acapulco, Mexico, supporting the equatorward flow of the CC near the coast. During May–June, the flow
around this gyre becomes less organized and the higher SSH begins to extend to the Mexican coastline near
Acapulco, interrupting the low SSH along the coast and possibly contributing to a poleward MCC near
the coast [Mascarenhas et al., 2004; Lavín et al., 2014]. This may result in the transport of a mixture of tropical
surface water (TSW) and CC water into the GC, increasing SSH in the GC. The July–August period of this SSH
climatology is reported in Figure 8b of Strub and James [2002], where relatively high SSH off Acapulco is
associated with stronger poleward geostrophic surface currents along the coast.
A more detailed view of this phenomenon is shown in Figure 2, which focuses on SSH and geostrophic
velocity fields near the GC entrance and farther south on 6 June 2012. We have studied the evolution of
SSH/geostrophic currents in this region during May–July for several years using the National Oceanic
and Atmospheric Administration (NOAA) CoastWatch website cited in section 2, and the SSH/current patterns
are consistent with Figure 1 and Strub and James [2002]. In late May the anticyclonic gyre off Acapulco
degrades with higher SSH extending toward the coast and developing along the coast, with relatively strong
poleward geostrophic currents emerging out of the warm pool by early June. It appears possible that these
surface currents may augment and accelerate the MCC as it enters the GC. Indeed, Lavín et al. [2014]
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4. documented an enhancement of a
poleward current on the shelf and slope
of the mainland side of the GC during
June 2004, having a mean speed
around 0.60 m sÀ1
with speeds up to
0.80 m sÀ1
, taking ~20 days for a
particular drifter to travel the 1000 km
from the GC entrance to its head. The
drifters and satellite images suggested
that this current enhancement
lasted less than 1 month. This is
roughly consistent with observations
from the NOAA CoastWatch website
that show the strongest poleward
geostrophic currents out of the warm
pool region (e.g., as in Figure 2) during
the month of June.
Examining all the reasons for the
change in SSH is beyond the scope of
this paper. SSH depends on ocean
surface currents, mesoscale eddies,
and the heat content of the water
(affecting water density and volume),
the former depending on the change in
the surface wind stress curl and wind
direction [Steward, 2008]. The
equatorward surface wind speed
maxima and wind stress curl maxima
off Baja California that occur during
March and April (the average wind
speed is 5–7 m sÀ1
, and the average
wind stress curl is 8–12 × 10À8
NmÀ3
)
are the reason for the strong CC.
However, equatorward wind speed and
wind stress curl decrease sharply
during late May and June which corresponds to the decrease in equatorward current velocities [Fiedler, 2002]
and the buildup of high SSH along the Mexican coast.
May is also the month when Collins et al. [1997] found maximum surface velocities at the entrance of the
GC and when Mascarenhas et al. [2004] measured a maximum in the poleward advection of TSW and heat
flux through the GC entrance. A buildup of higher SSH along the southern coast of Mexico and higher
SSH inside the GC continues through July [Strub and James, 2002], with poleward geostrophic currents
continuing to increase the heat content of the GC surface layer [Castro et al., 1994]. Other observations of
surface current velocities reinforce these results [Fiedler, 2002; Flores-Morales et al., 2009]. The above
description of TSW directed up the coast out of the warm pool region is consistent with the findings of
Kessler [2006] who observed no connection between the CRCC and MCC. Therefore, the rapid increase in
GC SSTs in late spring/early summer can at least partly result from the evolution of SSHs and associated
geostrophic currents as described above.
Poleward surface currents emerging out of the tropical eastern Pacific warm pool help to explain
the rapid increase of GC SSTs in May and June [Lavín et al., 2009], in sharp contrast to SSTs on the Pacific
side of the peninsula that are sometimes ~10°C cooler. Lavín et al. [2009] observed that during June
the CC and MCC join into the GC, and produce high-current velocities ~0.40–0.80 m sÀ1
in a narrow
across-gulf band of ~30 km between the surface and 500 m depth along the mainland coast of Mexico.
Numerical modeling showed two summertime cyclonic gyres in the GC, one in the northern GC and
Figure 1. Two-month SSH relative to Earth’s geoid with 2 cm contour
interval for (a) March–April and (b) May–June. Black arrows show the
direction of geostrophic currents (Plots courtesy of Professor Ted Strub
from Oregon State University).
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5. another south of the island Archipelago region that define the southern boundary of the northern
GC [Beier, 1997; Marinone, 2003].
M2002 observed a delay in the warming of the northern GC compared to the rest of the GC, which can
be caused by the Archipelago islands. These islands constrict the flow of sea water into and out of the
northern GC, increasing current velocities in this region [Beier, 1997]. The islands also produce shallow tidal
mixing of sea water, pumping heat in the surface layer into the water column [Paden et al., 1991]. Tidal
mixing was considered to be the main reason for the vigorous upwelling in the Islands region [Paden et al.,
1991; Simpson et al., 1994]; however, Lopez et al. [2006] showed that a deep mean flow contributes more to
the deep inflow and surface outflow at both ends of Ballenas channel. As a result, convergence forms at
the bottom and divergence produces at the surface, leading to the strong upwelling and low SSTs in this
region. The delay in the SST warming in the northern GC may be primarily responsible for the relative
delay of the NAM onset in AZ compared to NM and northwestern Mexico [M2002]. Increased solar insolation
near the summer solstice results in the decline of mixing and associated cooling in the northern GC
[Paden et al., 1991], and consequently, the northern GC becomes as warm as or warmer than the rest of
the GC by late July to early August. This condition is often associated with relatively heavy NAM rainfall in AZ.
It is suggested by M2002 that knowledge of the factors causing cooler northern GC SSTs can improve the
predictability of the NAM onset in AZ.
Although various observational and modeling studies showed numerous characteristics of the NAM, a
mechanistic understanding of the NAM is still elusive. In this study, we offer a partial mechanism that
addresses both small- and large-scale processes. Small-scale processes, contributing to the NAM onset
and development, are revealed by investigating the effect of GC SSTs on the inversion at the top of the
marine boundary layer (MBL) and consequently on the vertical extent of high relative humidity over the GC.
Large-scale processes, governing the evolution of the NAM anticyclone, are proposed using climatologies
of SST, outgoing longwave radiation (OLR), and 500 hPa geopotential height analyses. Data and methods
are described in section 2; section 3 discusses the local-scale mechanism; section 4 describes implications for
a synoptic-scale mechanism; and finally, conclusions are presented in section 5.
2. Data and Methods
Daily precipitation data for 2012 was obtained from NOAA Center for Satellite Applications and Research
(STAR) satellite rainfall estimates (i.e., Hydro-Estimator) which use infrared (IR) data from NOAA’s
Figure 2. SSH with respect to geoid (shades) in units of cm and geostrophic current velocities (blue arrows) in units
of cm s
À1
for 6 June 2012. Poleward current velocities emerging out of the warm pool often exceed 30 cm s
À1
and
appear to contribute to the MCC entering the GC.
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